"A subgroup" antigens are inherited, as are other ABO antigens with A1 being dominant over A2. Individuals who are phenotypically A1 may be genotypically A1O, A1A1, or A1A2. A phenotypically A2 individual may be genotypically A2A3. These alleles are passed to offspring in the same manner as other ABO antigens.

Weak variant forms of the B antigen (B3, Bx, and Bel) also exist, but are very rare.